The present invention relates to a method of applying an at least one-layer removable polymer coating on glass substrates, in particular display glasses or solid plate substrates, as a protective layer. The present invention also relates to a display glass,:and an electronic device, which includes the display glass. Moreover, the present invention also relates to hard disk substrates of glass or glass ceramic.
Glass substrates are substrates of conventional glass as well as substrates or glass ceramic. During the production of glass substrates, in particular for display glasses or solid plate substrate of glass or glass ceramic, there are massive yield problems, since the glass substrate due to treatment and transportation have defects in form of scratches and particles. These scratches moreover can induce glass breakage which acts destructively when occurred during a process step, since-the process must be interrupted for cleaning the machines from such chips which can cause further scratches.
During splitting and cross cutting, particles are released, and they lie on the glass surface and act as abrasive particles during the further transportation and treatment so as to lead to scratching of the surface. Also, the edge treatment which follows the separation of the substrate leads to loading of the glass surface with glass particles, which again act as abrasive bodies. Moreover, during the edge treatment for cooling and for removal of the grinding solution, a high pressure water jet is oriented to the edges.
Methods are known, in which at both sides polymer foil is laminated. Such methods have several disadvantages. The protective foil can be applied after the cutting of the glass and can therefore protect only during the transport and during the edge treatment. During the edge treatment the foil is partially lifted at the edges and under it a grinding dust is collected and the glass is damaged. Moreover, especially in thin glass, due to the required pulling force for removing the foil glass breakage again occurs in many instance.
For the lamination of the polymer protective foil, adhesives materials are required, which not always can be removed without resides. The glass surface must be polished thereafter in certain circumstances, to obtain the required quality. The post-polishing is performed mainly mechanically and removes the adhesive residuals, but on a microscopic plane does not lead to an optimal surface quality, as desired for example for display glass or optical special glass or also solid plate substrate, since by polishing minute scratches are produced. A great disadvantage is also the high refuse by glass breakage during polishing.
The German patent document DE 36 15 277 A1 discloses a method, in which the flat glass is provided with a scratch-resistant splinter protecting coating. Immediately after the manufacture, within the cooling region a synthetic plastic powder is applied, which is melted on the glass band. For increasing the mechanical stability, a base layer and a scratch-resistant cover layer are applied. The sprinter protective layer produced in this manner can not be again removed. In addition, no surface quality which is suitable for display is obtained. Utilization as a solid plate substrate is not possible due to the synthetic plastic layer.
The Japanese patent document JP-H 102 26537 discloses a method, in which a water-soluble protective film is applied, which does not affect the glass (it does not contain alkali salt) and is resistant against moisture. For the protective film, polyvinyl alcohol solutions in water are utilized, wherein the polyvinyl alcohol has an average molecular mass of at most 51,600 g/mol, preferably under 25,800 g/mol, and is partially hydrolyzed, preferably between 55% and 85%. The upper limits are based on that, with high average molecular masses as well as with hydrolysis degrees below substantially 95%, the water solubility dramatic drastically reduces. With low hydrolysis degrees, to the contrary, the water solubility increases with the hydrolysis degree. This protective film of polyvinyl alcohol with an average molecular mass of less than approximately 50,000 g/mol and a hydrolysis degree between 55% and 85% is sufficiently water soluble even below 30xc2x0 C. Such a polyvinyl alcohol protective film is removed during the edge treatment in the shortest time.
The patent document BE 714347 discloses a further process on the basis of a water soluble polymer protective film. In this process a mixture of hydroxylethycellulose and dialdehyde in water or of polyvinylalcohol and dialdehyde in water is used. The dialdehyde has the action of a crosslinking agent which makes the protective film swellable and reduces the adherence of the protective film to the glass. By the admixing of a dialdehyde, the water solubility of the polymer protective layer is adjusted. There is no disclosure as to the polymerization degree of the preferable polyvinylalcohol. The hydrolysis degree must be at 85% or higher. It has been however shown that the protective films of such polymers are removed during the edge treatment.
Accordingly, it is an object of the present invention to provide a method for protection of surfaces of glass substrates, in accordance with which the surfaces of glass substrates from the production to the end stages of the treatment, in particular the wet edge treatment, and during all intermediate transports are protected from scratching and abrasive bodies, which also can be performed with a lowest possible number of process steps, with which the refuse is minimized, and which guarantees the highest possible surface quality of the end product.
It is also an object of present invention to provide a display glass which at low manufacturing cost has an excellent surface quality, or an electronic device which is provided with such a display glass.
Moreover, it is also an object of present invention to provide a hard disk substrate, which has low manufacturing costs and an excellent surface quality.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of applying a detachable protective coating comprising at least one layer of polyvinyl alcohol on a glass substrate, in particular a display glass or a hard disk substrate in which the applied polyvinyl alcohol has an average molecular weight xe2x89xa755,000 g/mol and a hydrolysis degree xe2x89xa795%.
In accordance with another feature of present invention a method is provided, in accordance with which a polymer is utilized, which is soluble in polar organic solvent.
Still another feature of present invention resides in a display glass which before, during or after transport and/or complete treatment is not polished.
In accordance with a further feature of present invention a display glass is provided which has at least one removable polymer layer, and the at least one polymer layer is soluble in water at a temperature xe2x89xa750xc2x0 C., or in a polar organic solvent, or in a mixture of water and a polar organic solvent.
In accordance with still a further feature of present invention, an electronic device is proposed which has a display glass, and the display glass before, during or after the transport and/or complete treatment is not polished.
Also, a hard disk of glass or ceramic is disclosed, which before, during after transportation and/complete treatment is not polished.
In accordance with another feature of the invention, also a hard disk is disclosed, which is composed of glass or glass ceramic and has at least one releasable polymer layer, and the at least one polymer layer is soluble in water at a temperature greater than or equal to 50xc2x0 C. or in a polar organic solvent or in a mixture of water and a polar organic solvent.
Finally, it is proposed to use a glass or ceramic substrate as an initial blank for a hard disk used in electronic devices.
In accordance with a further method, at least one layer of polyvinyl alcohol is applied on the glass substrate. The polyvinyl alcohol has an average molecular weight of greater than or equal to 55,000 g/mol and a hydrolysis degree greater than or equal to 95%. The thusly produced at least one polyvinyl alcohol layer is difficult to dissolve in cold water and resists the wet edge treatment. Depending on the requirements, the glass substrate is coated on both sides or only on one side.
The coating can be removed by washing with water with a temperature 50xc2x0 C. and higher, in particular 60xc2x0 C. and higher, preferably 70xc2x0 C. and higher, and in particular preferably 80xc2x0 C. and higher. The higher the water temperature, the faster can be the coating released. The pH value is
Due to the at least one polyvinyl alcohol layer, the original surface quality of the glass substrate surface during the whole treatment process and a possible transportation is maintained satisfactory. The removal by washing with water of the at least one polyvinyl alcohol layer is on the one hand very safe, so that in very thin glasses very little glass breakage occurs, and, on the other hand, it is very thorough since the at least one polymer layer is completely removed.
By an additional post-polishing, the surface quality is reduced by microscopic scratches and waste due to glass breakage is increased. Post-polishing is dispensed with when the glasses are produced in accordance with the inventive method.
It has been determined in an advantageous manner that for the coating polyvinylalcohol with an average molecular weight xe2x89xa7100,000 g/mol, especially xe2x89xa7150,000 g/mol is preferable.
It also has been found advantageous to use polyvinyl alcohol with hydrolysis degree xe2x89xa797.5.
The water solubility at room temperature of polyvinylalcohol with high average molecular masses and very high hydrolysis degree is however extremely poor (quasi non soluble), the water solubility at high temperatures is to the contrary very good. Therefore, these very high polymerized polyvinylalcohols are suitable for protecting the glass substrate surface during treatment step of the moist edge treatment, during which for cooling and transportation of the grinding solution in cold, for example ambient temperature, high pressure water jet is oriented to the edge.
For the process it is technically advantageous to use aqueous solutions of 1 to 30 weight %, in particular 1 to 20 weight %, in particular preferably 5 to 10 weight % for applying of the polyvinylalcohol.
In a second method, at least one layer of a polymer is applied on the glass substrate, which is soluble in a polar organic solvent. Preferable are polar organic solvents, such as alcohol, ketone or ester, which are commercially available in great quantities at low prices. As for the polymer for the coating, preferably polyacrylate or polyvinylpyrrolidon/polyvinylacetate-copolymer or polyvinylcaprolactam.
The polymer coating is removed preferably with an alcohol, a ketone or an ester or an aqueous solution of its organic solvent. Particularly advantageous is isopropanol or ethanol or aqueous solutions of the same.
With the at least one polymer layer as well as with the polyvinyl alcohol coating, the original surface quality of the glass substrate surface is maintained As during the whole treatment process and possible transportation. The removal by washing with organic solvents, or aqueous solutions of the same, of the at least one polymer layer is, on the one hand, very safe,- so that in very thin glasses very little glass breakage occurs, and, on the other hand, is thorough, since the at least one polymer layer is completely removed.
Also, if the method were to include an additional post-polishing, microscopic scratches produced by the post-polishing would reduce the surface quality and glass breakage would be increased. The post-polishing is dispensed with in the inventive method.
In both processes, at least one polymer layer is applied preferably directly after the, production of the glass substrate onto the same, for conserving the surface quality of the manufacture before some abrasive or dirt particles can reduce, it. The application can take place directly on the glass train.
Depending on the performing process steps and application methods, the thickness of at least one polymer layer is set to between 0.1 to 100 xcexcm. A layer thickness of 1 to 15 xcexcm is particularly advantageous.
With regard to the application, there are diverse possibilities. Three preferable application methods are the immersion, the rolling and the spraying. The immersion is suitable for glasses which are already cut. The rolling is especially good for thick disks. The spraying to the contrary is preferably used for thin layers, and especially for glasses which have the highest surface quality and therefore are coated only on the glass train. In addition the spraying is contactless, so that the contamination of the glass substrate surface is maintained as low as possible. Preferably, the high volume low pressure (HVLP) process is utilized for this purpose, to obtain a maximum homogenous film and to retain as low as possible the overspray, or the overshooting spray fog.
When the glass band is coated not in warm condition (for example 90xc2x0 C.), the drying process must be enhanced. For this purpose on the one hand the infrared radiation, on the other hand hot air can be used. Both processes have in common that the glass is heated to a temperature of approximately 100xc2x0 C. Higher temperatures can lead to a modification of the glass layer or the glass. Lower temperatures reduce the drying speed.
During infrared drying the glass is heated with radiation. In particular radiation in a wavelength region between 3 and 10 xcexcm is especially recommended, as produced for example by gas heat radiators. Alternatively, electric heat radiators can be used.
During hot air drying, the glass band is acted upon at both sides with hot air. The drying can be performed by means of a cloud dryer, so that the glass band is held by the air stream in the cloud and the drier travels without contact.
When the coating must be removed again in shortest time, it is advantageous to enhance the washing with ultrasound. The sound waves propagate in the washing fluid and remove the coating mechanically.
In a preferable embodiment a polymer layer is applied exactly on the protecting glass substrate surface, which depending on the process is on the basis of polyvinylalcohols or a polymer which is soluble in a polar organic solvent. Thereby the expenses and the costs for this process stage are maintained low.
In special cases, in which the requirements to the protective coating are very high, it is preferable to apply at least two polymer layers which are different as to their solubility so that the layer applied directly on the glass substrate is more soluble than the layer located over it.
In a preferable embodiment, the glass substrate is applied first as a layer on polyvinylalcohol base and thereafter a layer of for example polyacrylate is applied. This has the advantage that the surface coating is very resistant against water and moisture, but both for the application and for the washing out of the coating less organic solvent is required than when an identically thick layer has only polyacrylate or a similar water soluble polymer and thereby the environment is affected less.
The invention also includes display glasses, which at no time are polished, as well as electronic devices, Which are provided with such display glasses. Moreover, the invention also deals with a display glass, whose coating can be removed with polar organic solvents or water at a temperature xe2x89xa750xc2x0 C. or aqueous solutions of polar organic solvents.
The invention also deals with a hard disk substrate made of glass or glass ceramic, which at no time is polished. Moreover, the invention deals with hard disk substrates, whose coating can be removed with polar organic solvents or water at a temperature xe2x89xa750xc2x0 C. or aqueous solutions of polar organic solvents. Also, the invention deals with use of such hard disk substrates as an initial blank for hard disks to be used in electronic devices.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its. construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the, accompanying drawings.